JSONInputStream: speed up by buffering the input

While our test suite is not speeded up tremendously by this (in this
developer's tests, in a best-of-five comparison, it was only about 7%,
but then, the unit tests' execution time is dominated by the apt runs),
projects with serious amounts of annotated classes should benefit from
this change (otherwise, the JSONInputStream would really read the JSON
byte by byte...).

Compared to the pre-JSON version of SezPoz, there is still a moderate
improvement of about 3-4%, proving at least that the switch to JSON
does not necessarily lead to a performance penalty.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>